The 18650 battery market has a wide range of applications, with the main popular being the 18650 lithium-ion battery. Other types of 18650 batteries have gradually begun to exit the market. So how to design the 18650 lithium battery protection board? This 18650 lithium battery protection board is not fixed, it is basically a customized battery protection board.
The protection function of lithium batteries is usually completed by the cooperation of protection circuit boards and current devices such as PTC. The protection board is composed of electronic circuits, which accurately monitor the voltage of the battery cells and the current of the charging and discharging circuit in an environment of -40 ℃ to+85 ℃, and timely control the on/off of the current circuit; PTC prevents severe damage to batteries in high-temperature environments.
A regular 18650 lithium battery protection board typically includes control ICs, MOS switches, resistors, capacitors, and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. Among them, the control IC controls the MOS switch to conduct under normal conditions, making the battery cell conductive to the external circuit. When the voltage or circuit current of the battery cell exceeds the specified value, it immediately controls the MOS switch to turn off, protecting the safety of the battery cell.
Under normal conditions of the lithium battery protection board, Vdd is at high level, Vss, VM is at low level, DO and CO are at high level. When any parameter of Vdd, Vss, VM changes, the level of DO or CO will change.
1. Normal state
Under normal conditions, both the "CO" and "DO" pins of N1 in the circuit output high voltage, and both MOSFETs are in a conducting state. The battery can freely charge and discharge. Due to the small conducting impedance of MOSFETs, usually less than 30 milliohms, their conducting resistance has little impact on the performance of the circuit. The current consumption of the protection circuit in this state is μ Grade A, usually less than 7 μ A.
2. Overcharge protection
The charging method required for lithium-ion batteries is constant current/constant voltage. In the initial stage of charging, it is constant current charging. As the charging process progresses, the voltage will rise to 4.2V (some batteries require a constant voltage value of 4.1V depending on the positive electrode material), and then switch to constant voltage charging until the current decreases. During the charging process of the battery, if the charger circuit loses control, it will cause the battery voltage to exceed 4.2V and continue constant current charging. At this time, the battery voltage will continue to rise. When the battery voltage is charged to exceed 4.3V, the chemical side reactions of the battery will intensify, which can cause battery damage or safety issues. In a battery with a protective circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, and different ICs have different values), the "CO" pin will change from high voltage to zero voltage, causing V2 to switch from conducting to turning off, thereby cutting off the charging circuit and preventing the charger from charging the battery, providing overcharging protection. At this time, due to the presence of the built-in body diode VD2 of V2, the battery can discharge external loads through this diode. There is a delay time between the detection of battery voltage exceeding 4.28V by the control IC and the issuance of the turn off V2 signal. The length of this delay time is determined by C3, usually set at around 1 second, to avoid misjudgment caused by interference.
3. Overdischarge protection
During the discharge process of the battery to external loads, its voltage will gradually decrease. When the battery voltage drops to 2.5V, its capacity has been fully discharged. If the battery continues to discharge the load at this time, it will cause permanent damage to the battery. During the battery discharge process, when the control IC detects that the battery voltage is below 2.3V (this value is determined by the control IC, and different ICs have different values), its "DO" pin will change from high voltage to zero voltage, causing V1 to switch from conducting to turning off, thereby cutting off the discharge circuit and preventing the battery from discharging the load again, providing over discharge protection. At this time, due to the presence of the built-in body diode VD1 of V1, the charger can charge the battery through this diode. Due to the fact that the battery voltage cannot be further reduced under over discharge protection, it is required that the consumption current of the protection circuit be extremely small. At this time, the control IC will enter a low-power state, and the power consumption of the entire protection circuit will be less than 0.1 μ A. There is also a delay time between the detection of battery voltage below 2.3V by the control IC and the issuance of the turn off V1 signal. The length of this delay time is determined by C3, usually set at around 100 milliseconds, to avoid misjudgment caused by interference
4. Short circuit protection
During the discharge process of the battery on the load, if the circuit current is large enough to make U>0.9V (this value is determined by the control IC, and different ICs have different values), the control IC will judge it as a load short circuit, and its "DO" pin will quickly change from high voltage to zero voltage, causing V1 to turn from conducting to off, thereby cutting off the discharge circuit and providing short circuit protection. The delay time for short-circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to overcurrent protection, but the judgment method and protection delay time are different. In addition to controlling the IC, there is also an important component in the circuit, which is the MOSFET. It plays a switching role in the circuit. As it is directly connected in series between the battery and the external load, its conduction impedance has an impact on the performance of the battery. When the selected MOSFET is good, its conduction impedance is small, the internal resistance of the battery pack is small, and the load capacity is also strong. During discharge, it consumes less electrical energy.
5. Overcurrent protection
Due to the chemical characteristics of lithium-ion batteries, battery manufacturers have specified that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery discharges beyond 2C, it will cause permanent damage to the battery or safety issues. During the normal discharge process of the battery on the load, when the discharge current passes through two series connected MOSFETs, a voltage is generated at both ends due to the conduction impedance of the MOSFETs. The voltage value U=I * RDS * 2, which is the conduction impedance of a single MOSFET, is detected by the "V -" pin on the control IC. If the load is abnormal for some reason, causing an increase in the circuit current, when the circuit current is high enough to make U>0.1V (this value is determined by the control IC, and different ICs have different values), the "DO" pin will change from high voltage to zero voltage, causing V1 to turn from conduction to turn off, thereby cutting off the circuit. The discharge circuit is adjusted to zero current in the circuit, providing overcurrent protection. There is also a delay time between the detection of overcurrent by the control IC and the issuance of the turn off V1 signal. The length of this delay time is determined by C3, usually around 13 milliseconds, to avoid misjudgment caused by interference. In the above control process, it can be seen that the magnitude of its overcurrent detection value not only depends on the control value of the control IC, but also on the conduction impedance of the MOSFET. When the conduction impedance of the MOSFET is larger, the overcurrent protection value for the same control IC is smaller.
Design considerations for 18650 lithium battery protection board:
1. Overcharge detection voltage: Under normal conditions, Vdd gradually increases to the CO terminal, changing from a high level to a low level, and the voltage between VDD-VSS.
2. Overcharge release voltage: In the charging state, Vdd gradually decreases to the CO end, changing from low level to high level, and the voltage between VDD-VSS.
3. Overdischarge detection voltage: Under normal conditions, Vdd gradually decreases to the voltage between VDD-VSS at the DO end, changing from a high level to a low level.
4. Overdischarge release voltage: In the over discharge state, Vdd gradually rises to the voltage between VDD-VSS at the DO end, changing from a low level to a high level.
5. Overcurrent 1 detection voltage: Under normal conditions, VM gradually rises to DO and changes from high level to low level, resulting in voltage between VM-VSS.
6. Overcurrent 2 detection voltage: Under normal conditions, the VM rises from OV at a speed of 1ms to 4ms and then changes from high level to low level at the DO end.
7. Load short circuit detection voltage: Under normal conditions, VM starts at OV and starts at 1 μ S above 50 μ The speed below S increases to the voltage between VM-VSS at the DO end from high level to low level.
8. Charger detects voltage: In an over discharge state, the voltage between VM and VSS gradually decreases from OV to DO from low level to high level.
9. Current consumption during normal operation: Under normal conditions, the current flowing through the VDD terminal (IDD) is the current consumed during normal operation.
10. Overdischarge consumption current: In the discharge state, the current flowing through the VDD terminal (IDD) is the overcurrent discharge consumption current.